Kerr Effect In Optical Fibers Research Articles

Inter-channel nonlinearity compensation in long-haul optical fiber communication using overlapped-dense LSTM network: Complexity reduction and visual understanding

The nonlinear distortion resulted from the Kerr effect in optical fibers limits the transmission capacity in long-haul optical fiber communications. In wavelength-division multiplexing (WDM) systems, the inter-channel nonlinear distortion is generally stronger than the intra-channel one, and also more difficult to be compensated, despite that there exist many reports on the nonlinearity compensation methods. Recently, recurrent neural networks have emerged as a potential solution without relying on detailed transmission link information. However, these networks often suffer from high computational complexity and lack interpretability due to their black-box nature. In this paper, we propose a novel overlapped-dense long short-term memory network (OD-LSTM). It can track the correlation of cross-phase-modulation (XPM) and consequently is capable of compensating the inter-channel nonlinear distortion. For a lower complexity, the overlapped-dense structure reduces the hidden units at time steps of different regions, since the correlation of XPM is reduced for the more distant symbols. To evaluate the effectiveness of our proposed scheme, we conduct a numerical simulation on a nine-channel WDM coherent optical transmission over a 1600-km standard single-mode fiber (SSMF) with DP-16QAM at 40 Gbaud. At the optimal optical power of −3 dBm, we observe a 0.52 dB Q-factor gain with the OD-LSTM equalizer, outperforming the digital back-propagation (DBP) with three steps per span and the Co-LSTM regressor. In terms of complexity, our proposed approach achieves a 13.5 % reduction compared to the Co-LSTM classifier, and its complexity is slightly lower than that of the ultralow complexity Co-LSTM regressor. We also discuss the visual understanding of the OD-LSTM, which provides insights into the internal mechanisms of the network.

Companding transform based SPM compensation in coherent optical OFDM transmission

We demonstrate a mitigation of fiber nonlinearity based on μ-law companding transform in coherent optical OFDM transmissions. High peak-to-average power ratio (PAPR) increases fiber nonlinear impairments caused by the Kerr effect in optical fiber. The μ-law companding modifies amplitude profile of OFDM signal with time domain signal processing, which reduces high PAPR of OFDM signal. The effects of companding parameter on noise enhancement and PAPR variation are presented. The impacts of companding transform on system performances are evaluated in a single polarization system as well as polarization multiplexed system. The resolution of analog-to-digital converter (ADC), dispersion map of transmission link, and launch power tolerance are also considered. The results of bit-error-rate (BER) measurements show that the μ-law companding improves OSNR margin over 5.5 dB after transmission of 1,040 km over SMF.

Optical orthogonal division multiplexing for long haul optical communications: A review of the first five years

Optical OFDM was proposed for dispersion compensation in long-haul optical communications systems in 2006 in two forms, one using direct-detection and the other using coherent detection. Since then there has been extensive innovation towards developing intermediate forms of optical OFDM that are more suited to specific applications. This review paper presents our view on the developments in optical OFDM for long-haul optical transmission applications. It covers the basic elements of radio OFDM before concentrating on direct detection optical OFDM and its development, followed by coherent optical OFDM. All-optical OFDM is then considered, together with optical methods of generating and separating the OFDM subcarriers. The paper then discusses the critical issue of nonlinear degradation due to the Kerr effect in optical fibers and reviews recent innovations to mitigate the effects of fiber nonlinearity. Finally some future research directions are discussed.

Capacity Limits of Information Transport in Fiber-Optic Networks

The instantaneous optical Kerr effect in optical fibers is a nonlinear phenomenon that can impose limits on the ability of fiber-optic communication systems to transport information. We present here a conservative estimate of the "fiber channel" capacity in an optically routed network. We show that the fiber capacity per unit bandwidth for a given distance significantly exceeds current record experimental demonstrations.

Experimental schemes for developing fiber-based source of entangled photon pairs

Based on the theory of third-order nonlinear Kerr effect in optical fiber, we experimentally demonstrate the generation of photon pairs by exploiting spontaneous four-wave mixing. The photon pairs are produced by pumping a fiber Sagnac loop and a piece of straight fiber with a pulsed laser, respectively. By comparing the experimental results, we analyze how to optimize the experimental parameters to obtain entangled photon pairs with high purity. Our results are useful for developing the all-fiber source of photon pairs and single photon source, which are well suited for quantum communication.

Nonlinear Phase Noise in Optical-Fiber-Communication Systems

Gordon and Mollenauer, in their famous paper published in 1990, laid out how the interplay between the nonlinear Kerr effect in optical fibers and the amplified spontaneous-emission (ASE) noise from the optical-amplifiers results in enhanced levels of noise and degrades the performance of modulation schemes that encode information in, particularly, the phase of the optical carrier. This phenomenon has been termed as nonlinear phase noise in the literature. In this paper, we first present a comparative and critical review of previous techniques that have been proposed for the analysis of nonlinear phase noise by forming a classification framework that reveals some key underlying features. We then present a unifying theory and a comprehensive methodology and computational techniques for the analysis and characterization of nonlinear phase noise and its impact on system performance by building on and extending previous work that we identify as most favorable and systematic. In our treatment, we consider a multichannel multispan optically amplified dense wavelength-division multiplexed system and develop general techniques for the analysis of the intricate interplay among Kerr nonlinearity, chromatic dispersion, and ASE noise, and for computing the bit-error-ratio performance of differential phase-shift-keying (DPSK) systems. By means of the extensive results we present, we demonstrate and argue that correlated noise behavior plays a most significant role in understanding nonlinear phase noise and its impact on DPSK system performance.

Verifying continuous-variable entanglement of intense light pulses

Three different methods have been discussed to verify continuous variable entanglement of intense light beams. We demonstrate all three methods using the same setup to facilitate the comparison. The nonlinearity used to generate entanglement is the Kerr effect in optical fibers. Due to the brightness of the entangled pulses, standard homodyne detection is not an appropriate tool for the verification. However, we show that by using large asymmetric interferometers on each beam individually, two noncommuting variables can be accessed and the presence of entanglement verified via joint measurements on the two beams. Alternatively, we witness entanglement by combining the two beams on a beam splitter that yields certain linear combinations of quadrature amplitudes which suffice to prove the presence of entanglement.

All-optical ultrafast nonlinear switching utilizing the optical Kerr effect in optical fibers

An ultrafast switching technique utilizing the optical Kerr effect in optical fibers is described and its switching characteristics are considered, taking into account chromatic and polarization dispersion. Experimental results on picosecond optical Kerr modulation and 2 Gb/s all-optical complete demultiplexing of a 30 ps probe pulse train, as well as all-optical chirping manipulation of ultrashort optical pulses, are presented. Applications of the optical Kerr switching technique to the determination of the nondiagonal component n281 ofthe nonlinear refractive index in nonlinear optical media and to the generation of ultrafast pulse trains are also discussed.